When a guy with a girlfriend clearly likes you, and you like him, what are you supposed to do?

I know this sounds bad. But I can tell he likes me, despite the fact that he's had a girlfriend for the entire time I've known him. He's very nervous around me, and stares at me all the time. When he brought his girlfriend into work, he seemed to pretend he didn't know her.. he walked ahead of her like she was a stranger. He's told me that he's going on vacations with his family, but he actually went with her, so he's clearly trying to hide the fact that he has a girlfriend from me. I just love him so much. What am I to do?